The present invention relates to a process and an apparatus for controlling a composition of a chemical copper plating solution containing copper ions, a reducing agent, a complexing agent, and a pH-adjusting agent as essential components, and more particularly to a process and an apparatus for continuously and automatically controlling a pH, a concentration of a reducing agent, a concentration of copper ions and a concentration of a complexing agent in a chemical copper plating solution.
When a surface of ABS resin is subjected to a bright plating for decoration, it is the ordinary expedient to carry out a copper substrate plating by chemical copper plating to give an electroconductivity to the surface of the resin. In that case, the mechanical strength of a substrate plating film has been neglected, and thus a composition of a chemical copper plating solution, which gives a great influence upon the substrate plating film, has been only intermittently controlled.
However, recently, conductors have been more often formed directly on the insulating base plates by chemical copper plating. In that case, not only an electro-conductivity, but also mechanical characteristics of the chemical copper plating films have been regarded as important. The mechanical characteristics of a chemical cooper plating film depend upon concentrations of main components in a chemical copper plating solution, and thus it is very necessary to continuously control the composition of a chemical copper plating solution.
A continuous control of a composition of a chemical copper plating solution has been so far carried out as to pH, a concentration of a reducing agent, a concentration of copper ions and a concentration of a complexing agent in the following manner as given by (a) to (d).
(a) pH (Japanese Laid-open Patent Application Specification Ser. No. 44434/78): PA0 (b) Concentration of a reducing agent (Japanese Laid-open Patent Application Specification Ser. No. 65226/78; Japanese Patent Publication No. 32121/70): PA0 (c) Concentration of copper ions (Japanese Laid-open Patent Application Ser. No. 44434/78): PA0 (d) Concentration of a complexing agent (Japanese Laid-open Patent Application Specification Ser. No. 44434/78):
A pH of a chemical copper plating solution is measured as a potential by means of a pH meter using glass electrode-calomel electrode, and when there is a difference in potential between the chemical copper plating solution and a fresh chemical copper plating solution, the pH meter emits a signal to drive a control unit, to make up the chemical copper plating solution with a supplementing solution. The pH value can be kept thereby equal to that of the fresh chemical copper plating solution.
Generally, formaldehyde is used as a reducing agent. According to one mode of practice, a concentration of formaldehyde in a chemical copper plating solution can be kept constant by diluting the plating solution with a known amount of a solution, adding a material capable of coloring through reaction with formaldehyde (coloring agent) in excess thereto, measuring a degree of coloring by a colorimeter or a spectrophotometer, for example, by measuring an intensity of transmitted light by means of a phototube and converting it to an electric signal, and driving a control unit according to the electric signal, when required.
According to another mode of practice, a concentration of formaldehyde in a chemical copper plating solution can be kept constant by sampling the chemical copper plating solution, adding sodium sulfite in excess thereto, thereby reacting it with formaldehyde in the sampled plating solution, measuring a change in pH of the sampled plating solution by means of pH meter of the above-mentioned type, since the pH of the sampled plating solution is increased through the reaction, converting the change in pH to an electric signal, and driving a control unit according to the electric signal, when required.
According to other mode of practice, a concentration of aldehyde in a chemical copper plating solution can be kept constant by plating a piece of metal having a given area such as copper, platinum, gold, palladium, etc. in a sampled chemical copper plating solution, measuring a turbidity of the sampled plating solution saturated with a hydrogen gas generated proportionately to the amount of consumed aldehyde by means of a turbidimeter, converting it to an electric signal, and driving a control unit according to the electric signal, when required.
According to further mode of practice, a concentration of aldehyde in a chemical copper plating solution can be kept constant by detecting a concentration of copper ions in the plating solution according to the following manner as described in item (c), since a molar ratio of consumed formaldehyde to copper ions is usually in a range of 2-3, supplementing the copper ions by an amount of consumed copper ions as detected and at the same time adding formaldehyde thereto in an amount 2-3 times as many as the supplemented amount of copper ions.
A concentration of copper ions in a chemical copper plating solution can be kept constant by adding to the chemical copper plating solution Cu.sup.+2 in a form of an aqueous CuSO.sub.4.5H.sub.2 O solution in a very small excess of the amount necessary for the reaction with a total amount of a free complexing agent, for example, EDTA, contained in the chemical copper plating solution, measuring an equilibrium potential between Cu.sup.+2 and a very small amount of impurity Cu.sup.+ present in the plating solution, thereby determining an amount of EDTA in the plating solution, indirectly determining an amount of Cu.sup.+2 since a ratio of Cu-EDTA compound to free EDTA is constant in the plating solution (a reducibility of a chemical copper plating solution greatly depends upon a ratio of Cu.sup.+2 /a complexing agent, that is, a stability of Cu.sup.+2 at the complex formation, and EDTA is usually added at a concentration 1.5 to 5 times as high as the concentration of Cu.sup.+2), emitting a difference in potential from the set potential as a signal, when the measured potential is found to differ from the set potential, and driving a control unit according to the signal.
Compounds of copper and complexing agents such as EDTA in a chemical copper plating solution can be decomposed by making the pH of the chemical copper plating solution lower than 1, thereby bringing all the amount of the complexing agent into a free state. Then, Ti.sup.+3 is added to the plating solution. When the amount of Ti.sup.+3 is a little lower than the concentration of Cu.sup.+2 in the plating solution, an oxidation-reduction potential of copper ions can be determined, for example, by measuring a potential by a platinum-calomel electrode. When the amount of Ti.sup.+3 added is a little larger than the concentration of Cu.sup.+2, the oxidation-reduction potential of titanium ions can be determined. The difference in potential represents an amount of copper ions in the plating solution. Since a ratio of the concentration of copper ions to the amount of a complexing agent in a fresh chemical copper plating solution is constant, the concentration of a complexing agent can be indirectly determined thereby. When the measured potential is found to differ from the set potential, a difference in potential from the set value is emitted as a signal to drive a control unit, and the concentration of a complexing agent can be kept constant thereby.
However, the continuous control of a composition of a chemical copper plating solution in the manner as described in the foregoing items (a) to (d) still has the following disadvantages (i) to (iv).
(i) pH:
When the pH of a chemical copper plating solutoin is measured by means of glass electrode-calomel electrode, an error in measurement is significant, because the plating solution has a pH of 11 to 13, and OH.sup.- ions are adsorbed on a glass wall of the electrode, so that the adsorption of H.sup.+ ions becomes difficult, and also the accuracy in the measured potential is deteriorated with time, because the glass wall of the glass electrode is gradually attacked by the chemical copper plating solution itself.
Beside the glass electrode, an antimony electrode, and a quinhydrone electrode are known.
However, all of these electrodes are slow in response speed, and troublesome in handling, and thus are not suitable for the continuous control of pH of a chemical copper plating solution.
(ii) Concentration of a reducing agent:
According to the colorimeter or the spectrophotometer, the difference in the intensity of transmitted light due to the different degree of coloring is measured as an electric current by a phototube, and is displayed as a difference in potential, and thus the necessary apparatus will be very expensive. Furthermore, a response speed is so low that the time for measurement is inevitably prolonged, and an accurate control cannot be attained.
When the change in pH of a chemical copper plating solution is measured by adding sodium sulfite to the plating solution for the reaction with formaldehyde, there appears the same disadvantage as mentioned in the foregoing item (i).
When the piece of metal is plated to determine the amount of generated hydrogen by the turbidimeter, the turbidimeter itself is expensive for the same reason as mentioned above, and also the hydrogen gas is not uniformly dispersed in the plating solution, so that the measurement cannot be made accurately. Furthermore, the hydrogen gas cannot be rapidly separated from the plating solution, resulting in a low response speed.
The control of the amount of formaldehyde in view of the measurement of the amount of consumed Cu.sup.+2 has the disadvantage as described in the following item (iii), and also it is impossible to supplement the amount of formaldehyde consumed by evaporation and self-decomposition.
(iii) Concentration of copper ions:
The change in potential when Cu.sup.+2 starts to form in a chemical copper plating solution is so small, that the measurement accuracy will be about .+-.5%, and rather will be much lowered down to .+-.13% when the plating solution is repeatedly used, on account of the action of HCOO.sup.-, CO.sub.3.sup.- and SO.sub.4.sup.-2 accumulated in the plating solution.
(iv) Concentration of a complexing agent:
The pH control is essential, and the solution containing Ti.sup.+3 is not stable. Thus, the solution cannot be used for a prolonged time, and also a compound liberting Ti.sup.+3 is expensive.
The conventional apparatus for controlling a chemical copper plating solution comprises (1) a sampling means of continuously sampling a definite volume of a chemical copper plating solution from a plating tank, (2) a pH meter of measuring a pH of said definite volume of the plating solution sampled by the sampling means by a platinum-calomel electrode, and a means of supplying a pH-adjusting solution to the plating tank according to the measured pH value, (3) a means of supplying Cu.sup.+2 ions in a very little excess of the amount of free EDTA present in the plating solution to the plating solution leaving the pH meter, a means of measuring an oxidation-reduction potential of copper ions in the plating solution leaving the Cu.sup.+2 ions-supplying means, thereby determining the amount of copper ions, and a means of controlling copper ions to be supplied to the plating tank according to the determination of the amount of copper ions, (4) a means of acidifying the definite volume of the plating solution sampled by said sampling means, a means of supplying Ti.sup.+3 ions in a very little excess of the amount of Cu.sup.+2 ions present in the plating solution to the plating solution leaving the acidifying means, a means of measuring an oxidation-reduction potential of titanium ions in the plating solution leaving the Ti.sup.+3 ions-supplying means, thereby determining the amount of a complexing agent, and a means of controlling the complexing agent to be supplied to the plating tank according to the determination of the amount of complexing agent. However, said conventional apparatus for controlling the composition of a chemical copper plating solution has said disadvantages (i), (ii) and (iv), and also the control of a concentration of a reducing agent cannot be effected according to the conventional apparatus. This is another great disadvantage.